big new day in this presidential race turbo charged by mitt romney's brand-new running mate. Congressman paul ryan bounded on to the campaign trail today. The crowds were raucous and he led the trail against the president. The president rallied the crowd of his own. It's "your voice your vote." The team is out enforce around the country on the candidates, the choices and george stephanopoulos is standing by with the bottom line on the next move. But david muir will start us off right now on the presidential race in florida. David. Reporter: Diane, this brand new republican ticket is off and running. Mitt romney speaking here in miami just a short time ago. It did not take him long to send paul ryan out to another battleground. That this was a real game-changer. Mitt romney in what many believe is the must-win state of florida Celebrating his new pick. I am delighted to have picked my running mate, paul ryan, a great leader. Reporter: But where was paul ryan? The same place the president was headed, iowa. Hey, nice to meet you. Yeah. Reporter: A senior adviser on the romney campaign telling me today paul ryan is a good match for the president. I heard that president obama starting his tour today and i heard he wasn't going to come to iowa state fair. But it was not long before ryan was interrupted. This protester almost immediately bringing up paul ryan's controversial budget plan that would change medicare into a voucher-like system that you would use to shop around for your own health care. One woman trying to climb on that famous iowa soapbox, removed acquittedly security. And what they might ask the president instead. You might ask the president the same question what I'm getting asked, that is, where are the jobs mr. President. Reporter: The fields are bone dry from the drought. The president with his own volley, telling them they should ask paul ryan about the stalled relief. Paul ryan is one of the leaders in congress standing in the way. Reporter: Paul ryan right against the president. And chose changes to medicare, a plan that they applauded. Governor romney you said later this year of paul ryan's latest proposal that it's absolutely right on. I'm curious, is there anything you disagree with it? We haven't ne by piece by piece, saying this is a difference. Even in the exact same party that have the same issues. But my plan for medicare is similar to his plan for medicare. Governor romney going on to tell me that neither he nor paul ryan would want to change medicare for retirees or near retirees if this is for a future generation to look at. But the fact that they are taking only this real policy difference with the president a sign this race is quickly

This transcript has been automatically generated and may not be 100% accurate.

Paul Ryan Heckled at Iowa State Fair

Hecklers to Ryan: "Stop the war on the poor," and "Hands off Social Security!"